1. Field of the Invention
This invention relates to a composite chromium plating film containing hard particles in a network of cracks formed on the hard chromium plating film and to sliding members covered with said film such as vanes in compressors or piston rings in internal combustion engines.
2. Description of the Related Art
Dispersing hard particles within a hard chromium plating film has been attempted to improve the wear resistance of the chromium plating film. Methods proposed up until now for dispersing hard particles within the hard chromium plating film are as follows.
A pulse electrolytic method (Japanese Patent Publication No. 59-028640) electrolyzing repetitively utilizing a comparatively low current density for codeposition of the hard particles and, a current density at which the chromium plating electrically deposits at normal speed.
A method (Japanese Patent Laid-open No. 61-003895) adding rare earth elements or compounds of rare earth elements to the chromium plating bath.
A method (Japanese Patent Laid-open No. 62-120498) adding self-lubricating particles and hard particles to a trivalent chromium plating bath.
However, these methods have drawbacks in that none of these methods are practical to use and these methods prove difficult to reproduce in our follow-up testing. Further problems are that the dispersion ratio was extremely low or the plating speed is extremely low.
The purpose of the composite chromium plating films mentioned above is to uniformly disperse hard particles into the hard chromium plating film. There is another method using a network of enlarged cracks containing hard particles in the hard chromium plating film (Japanese Patent Laid-open No. 62-56600). This method provides a composite chromium plating with satisfactory plating speed, control of the dispersion ratio and reproducibility.
The items disclosed in the Japanese Patent Laid-open No. 62-56600 are as follows.
Composite chromium plating film thickness: 10-100 .mu.m PA1 Crack width: 0.5 .mu.m or more, and even 1 .mu.m or more PA1 Hard particle size: 0.5 to 15 .mu.m (0.5 to 5 .mu.m in the embodiment) PA1 Hard particles: WC, Al.sub.2 O.sub.3, SiC, Si.sub.3 N.sub.4, BC, diamond PA1 Film thickness: 100-200 .mu.m PA1 Film hardness: Vickers hardness 850 to 1000 PA1 Hard particles: Al.sub.2 O.sub.3 (pulverized particles) PA1 Hard particles average size: 4.5 .mu.m PA1 Hard particles dispersion ratio: 5 percent by volume
This composite chromium plating film is actually utilized for piston rings in certain low-load diesel engines in Europe. The specifications for this commercially utilized composite chromium plating film are as follows.
However this piston ring covered with the above mentioned composite chromium plating film possesses excellent self-wear resistance and scuffing resistance, but wears down the mating material by large amounts and causes problems by drastically increasing wear in the cylinder bore. Accordingly the piston ring formed with the above mentioned composite chromium plating film is unsuitable for use in high load diesel engines or gasoline engines.
The type of hard particles and their total content in the above mentioned composite chromium plating film is thought to influence the amount of wear on the mating material, but Japanese Patent Laid-open No. 62-56600 makes absolutely no mention of this. Further, unlike the composite plating of nickel or Ni--Co--P types used in the conventional art, the above mentioned composite chromium plating has a different hard particle distribution and matrix metal, so alleviating the wear on piston ring mating material by utilizing the available knowledge of composite platings in the conventional art is impossible.